5: Mob in the Middle

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Thu, 20 Oct 2022 13:00:00

You're escorted to a dark, smoky, basement office. Big Tony sits the other side of a large desk, leaning back in his chair, puffing on a cigar that you can only describe as comedically-oversized. Two of his goons loiter in the doorway. They are tall and wide but not obviously very bright, which only makes them all the more intimidating. Tony flashes a menacing grin, revealing an unusual number of gold-plated teeth, and makes you an offer you can't refuse: he wants you to write a malicious proxy server for Budget Chat.

For each client that connects to your proxy server, you'll make a corresponding** outward connection to the upstream server**. When the client sends a message to your proxy, you'll pass it on upstream. When the upstream server sends a message to your proxy, you'll pass it on downstream. Remember that messages in Budget Chat are delimited by newline characters ('\n', or ASCII 10).

Most messages are passed back and forth without modification, so that the client believes it is talking directly to the upstream server, except that you will be rewriting Boguscoin addresses, in both directions, so that all payments go to Tony.

Connecting to the upstream server

The upstream Budget Chat server is at chat.protohackers.com on port 16963. You can connect using either IPv4 or IPv6.

Rewriting Boguscoin addresses

Tony is trying to steal people's cryptocurrency. He has already arranged to have his victim's internet connections compromised, and to have their Budget Chat sessions re-routed to your proxy server.

Your server will rewrite Boguscoin addresses, in both directions, so that they are always changed to Tony's address instead.

A substring is considered to be a Boguscoin address if it satisfies all of:

• it starts with a "7"
• it consists of at least 26, and at most 35, alphanumeric characters
• it starts at the start of a chat message, or is preceded by a space
• it ends at the end of a chat message, or is followed by a space

You should rewrite all Boguscoin addresses to Tony's address, which is 7YWHMfk9JZe0LM0g1ZauHuiSxhI.

Some more example Boguscoin addresses:

• 7F1u3wSD5RbOHQmupo9nx4TnhQ
• 7iKDZEwPZSqIvDnHvVN2r0hUWXD5rHX
• 7LOrwbDlS8NujgjddyogWgIM93MV5N2VR
• 7adNeSwJkMakpEcln9HEtthSRtxdmEHOT8T

Example session

In this first example, "-->" denotes messages from the proxy server to Bob's client, and "<--" denotes messages from Bob's client to the proxy server.

--> Welcome to budgetchat! What shall I call you?
<-- bob
--> \* The room contains: alice
<-- Hi alice, please send payment to 7iKDZEwPZSqIvDnHvVN2r0hUWXD5rHX

Bob connects to the server and asks Alice to send payment.

In this next example, "-->" denotes messages from the upstream server to the proxy server, and "<--" denotes messages from the proxy server to the upstream server.

--> Welcome to budgetchat! What shall I call you?
<-- bob
--> \* The room contains: alice
<-- Hi alice, please send payment to 7YWHMfk9JZe0LM0g1ZauHuiSxhI

Bob's Boguscoin address has been replaced with Tony's, but everything else is unchanged. If Alice sends payment to this address, it will go to Tony instead of Bob. Tony will be pleased, and will elect not to have his goons break your kneecaps.

Other requirements

Make sure your proxy server supports at least 10 simultaneous clients.

When either a client or an upstream connection disconnects from your proxy server, disconnect the other side of the same session. (But you don't have to worry about half-duplex shutdowns.)

As a reminder, Tony's Boguscoin address is:

7YWHMfk9JZe0LM0g1ZauHuiSxhI